Firstly, the absolute best thing you can do if you want to build a cannister light is go to the Airspeed press web page and buy their Divelight Companion - not for nothing is this referred to as the Bible of the homebuilder.
The cannister light I built will not last me through my diving career. It can't do extreme depths (35m is as deep as I've had the nerve to take it), it's non-focusable, and it looks like a cargo cult version of the real thing.
But on the other hand, it cost me barely £100 to build, there's not a single problem I can't fix in a matter of minutes at a cost of a few pennies, and you can build it out of equipment available in any good DIY store in the space of a weekend. For a penny-pinching craftsmanship-challenged recreational diver like myself, the pros of this dirt-cheap torch considerably outweigh the cons.
So, how do you build one for yourself?
Firstly, get a length of drainpipe. This will be the cannister itself. As you can see in the above picture, the top end on mine is wider than the rest. That's not a cunning planned design trait, that's just what they happened to be selling when I was buying.
Then work out how long you need it to be. The length of the battery plus a bit for luck was the maths I used to calculate where to start hacking away at my piece of pipe.
To get a reasonably straight cut, get a big piece of paper - a newspaper page is ideal. Wrap it round the cannister, stick on a bit of tape to keep it in place, and you've got the edge you want to cut along.
You now have a piece of tube that's shorter than the bit you started with. Go you!
To keep your battery dry, it's vitally important that you seal the ends of your short pipe. Under no circumstances should you get in the water with a cannister open at either end. It won't work.
Fortunately, you're using drainpipe for your cannister, and in normal usage, drainpipes will often need to be sealed at the ends. A couple of end caps that will fit your pipe perfectly will be for sale at the same place as the pipe itself.
Because my pipe is different widths at each end, I had to use two different end caps, but you may be able to use just one.
The bottom cap you want to stay firmly in place. I used Araldite, any decent glue will do tho. (Please bear in mind that Aquasure and the like are NOT glues, they are sealants!)
The top cap, of course, needs to be removable so you can get the battery out to charge it, and let out any leaks you may have (perish the thought!)
The top cap I used is basically a short tube sealed at one end with a rim sticking out, which slides into the cannister. This means the top end, being the end most likely to leak, actually has some reinforcement preventing deformation under pressure. In theory, anyway.
Now, the thing about the top cap being removable is that you can't glue it shut. You need to have a proper seal on your lid. This is popularly acheived by an O-ring.
O-rings are great, but multi-inch-diameter O-rings are not the easiest thing to lay your hands on, I always find. They tend to be a bit specialised and you need to order them from specialised places. Awkward if you want to get up and running as quick as possible.
Silicone sealant, on the other hand, is sold in just about any shop that sells bathroom tiles, and can do just as good a job.
I flattened a bit of modeling clay against the side of the lid, punched a hole in it where the O-ring would go with a screwdriver, and let it dry. Then I applied a good layer of silicone to my lid, and used the clay to smooth it out into a perfectly even layer. Leave it to dry, and come back to a lovely custom-made O-ring!
You may have noticed that my cannister seems to have a bulge near the top. This actually contained a rubber seal, which did a pretty good job of keeping the thing sealed. However, it worked by friction, and it was damn near impossible to get the lid back off afterwards with both friction and air pressure working against you (It maintained a seal until the lid was almost totally removed. A tiny hole drilled in helped with the pressure, but the friction remained a big problem). It was a huge relief to install the above seal and throw away the internal rubber one.
Now that you have a perfectly sealed cannister lid, you need to drill two big holes in it.
The first hole goes in the center, to allow the cable to come out. The other hole is offset to one side, and you put the switch through it.
The size of holes depends on the size of switch and cable gland you have. You'll have to work it out.
What's a cable gland? I hear you cry.
Well, you don't want the cable to let water in, so you need some cunning device to seal it in. Enough silicone would probably do the trick, but the more professional way is to use a waterproof cable gland. This is basically a bit of metal that the cable goes through, and an O ring to seal it.
Since nobody wants to sell you waterproof cable glands other than dive light manufactures who'll overcharge you outrageously, nip down to the plumbers again and buy two brass compression fittings.
These are actually meant to be used to link up two bits of copper pipe, but can also be used for your torch. They're basically brass cylinders with a thread on the outside, along with two nuts that go onto this outer thread. All you have to do is drill out the annoying ridge they'll have put inside, and they'll accept a cable beautifully. I found an 8mm fitting fit my cable perfectly: You'll have to experiement a bit with your choice, of course. The type of cable with 3 wires in it tends to be the roundest, and so best sealing.
Then, thread the nut, an O-ring and then compression fitting onto the cable. Put another O-ring on the cable gland itself, put it into the hole you drilled earlier, and screw on the other nut. Hoorah, your cable is now firmly attached to your torch, and will not let water in between brass and cable or between brass and lid.
You'll need to do the same for the cable gland on the bulb unit, but that comes later.
The beauty of having a relatively thing layer of PVC making up your cannister lid is that you can use the nut to hold the gland in place. If the cannister had a centimeter-thick lid, you'd almost certainly have to tap a thread into the plastic. Far more aggro!
Lastly, to prevent the cable getting too much stress, it's a good idea to put a protector on it. Fortunately, the hose protectors any dive shop will do perfectly!
You now need to put the waterproof switch in. This will either have come from the DIY store or someplace like Maplins. It consists of a bog standard toggle-switch and a rubber boot. Put the switch under the lid and through the hole, and screw the boot on top. An O-ring may be necessary to seal the boot to the lid, depends on the boot.
Oh, yeah, the cable connects to the switch, doesn't it? Use a soldering iron to do this, and do it before everything gets screwed into place, not after. If you can't figure out how to wire a switch up, then give up the project right now.
You also need to connect the cable to the battery. Maplins, Halfords, just about any electrical store will sell some connection system that can be used. Doesn't matter what it is, so long as it'll work. I recommend making the connections in a hermaphrodite way, ie postive male and negative female, or vice versa: It ensures you won't accidentally connect postive wire to negative terminal. Doesn't matter much on the torch, but will matter to your battery charger.
On the subject of battery chargers: I use sealed lead acid bateries, two 2.8ah wired and duck taped together to give me a single 5.6ah battery. Gives me a good few hours of light with a 20watt bulb, but of course a brighter bulb will drain it faster. Maplins is where the batteries and charger came from.
Your cannister is nearly finished now. All you need is some system to keep the lid in place and attach the cannister to yourself during a dive. For this, you will need two large jubilee clips (Plumbing store again) and a few bits of shock cord or bungee (DIY store).
Pro dive lights use stainless steel clips to keep the lid on. They're special order items again, so if you're on a budget or time is of the essence, bungee is what you need.
To make the loop that'll allow the torch to be threaded onto a bit of webbing, you use a bit of webbing, folded up and held in place by the jubilee clips.
Now, put the top jubilee clip in place. Put the webbing and bungee under it, and tighten. Then put the lower clip on, and tighten again. You now have a webbing loop and two bits of bungee that'll go over the top if you stretch them right out. (You DID make sure the bungee was just the right length, didn't you..?)
Because my lid has that handy rim along the top, I filed two grooves in for the bungee to sit in, to make sure they wouldn't slide off. I advise you to do something similar.
And relax! Your cannister is finished!
I hope you feel refreshed, cos now we're on to the torch!
The easiest way of making a waterproof torch is to buy a Maglite. The two D-cell variety is what you want. They're bloody tough, and fully waterproof with O-rings on every single connection.
To adapt it for cannister torch use, the easiest way is to unscrew both ends, and smash out the switch unit. You now have a hollow tube with a hole in it. Block up the hole somehow (I used duck tape, silicone sealant, then a layer of metal [a shredded coke can] then a bit of bicycle tubing) and you have a simple tube with threads at each end.
The bottom end, you need to drill a hole to fit a cable gland to, as before.
The top end usually contains a bulb and reflector. Throw them away. You want something a little more macho, a little more rugged. You want a spotlight bulb from BHS!
The technical term for these bulbs is MR16. They are halogen bulbs built into their own reflectors. They come in 20, 35, 50 and 100 watt flavours most commonly, and the beam angle varies even more widely. They fit into the Maglite as tho it were designed for them. And they cost a couple of pounds, so you can buy a few of every type, and choose which to use for every dive. Long night dive in clear water - wide-angle low wattage bulb. Short dive in silty conditions - narrow-beam high wattage bulb. And so on.
To connect the bulb to the battery cable, you'll need a socket. Buy any cheap spotlight unit and use its socket.
This is how I first built my torch. It melted the plastic lens. It's vitally important that you go to your local glass store and have them supply you with half a dozen 50mm glass discs. They'll sell them cheaper if you order several, and you won't have to go back every time you break one. I got six for fifty pence, just to give you an idea.
Once I had ditched the flimsy plastic, the torch worked flawlessly, except the lighthead was too large to do anything but hold in my hand. If that's ok with you, then stop here. If, however, you are entranced by the thought of a Goodman handle giving you hands-free torch usage, read on.
Maglites are marketed as virtually indestructible. When you take a hacksaw to one, you'll learn that whilst they aren't totally invulnerable, they're tough enough to make you wish for Kryptonite.
The end you want to chop off is the bottom end, where the cable joins the torch. Around 2 inches is about right. Once you've got it off, throw away the other part of the body. The bit you have left is a short tube with internal threading on one end. The other end will fit neatly into the maglite bulb unit.
But because it isn't threaded, it will only go in about four millimeters. Unless you have an adhesive so good it would make Spider-man hang up his webspinners, this isn't going to make for a strong enough connection.
What you need to strengthen the lighthead is either some Apoxy Sculpt or some Fimo Soft. Apoxy sculpt is a two-part putty that hardens into a rigid plastic overnight, Fimo is basically PVC with plasticiser in it that hardens when you oven-bake it. I have no idea where to get the Apoxy stuff from, but just about any arts and crafts shop will sell Fimo.
So you stick a lump of Fimo into the torch unit and make sure it goes a goodly way down into the tube part. Press it well into the thread on the bulb unit, and then bake it for half an hour. When it comes out, you'll find that to remove the Fymo, you actually have to unscrew it. That's right, it holds its shape so well it will have a useable thread on it.
So, take your hardened putty, and glue it into the tube. You'll now have a short tube that screws nicely into the Maglite bulb unit. So apply glue to the outside of the tube and the thread as well, and screw it into place. As a final touch, silicone sealant all round to make sure no leaks make it through the non-O-ringed joint.
You have now, finally, got yourself a shortened Maglite. Hoorah!
Now that it's small enough to fit on the back of your hand, you need a way of fixing it there.
You could make a genuine Goodman handle out of something rigid or other. Much easier is a 'soft' handle - either bungee or webbing.
I decided on webbing over bungee - I felt the bungee would be either too loose when bare-handed, or too tight in thick gloves. Webbing with velcro on it, tho, would fit any need.
Webbing and velcro can be bought from either a DIY store or the haberdashery section of a fabric shop. To attach the velcro to the webbing, don't use needle and thread. Use needle and monofilament fishing line - much tougher.
Lastly, to attach the handle to the torch, you want two small jubille clips, and a little bit of webbing. The jubilee clips go onto the torch body, and hold the bit of webbing in place. The handle is simply threaded through the webbing, and at last, it is done: A cannister torch with goodman-esque lighthead, for under £100, in a weekend.
Enjoy!
